CAPTCHA and INPUT TEXTBOX

currently i am working on a webpage designing project. I have to include CAPTCHA in my webpage. My doubt is , usually in text boxes , users will be able to get already entered values by just pressing the arrow key. In the case of CAPTCHA this must be disabled( in text box were I receive the user input for validation) How will I make this property set in a text box?? Any kind of suggestion would be of great help.

i has placed captcha to my contact form and has written styles to the input field for the value to enter catcha code but the styles are effected on chrome but not in firefox and ie; can any one suggest to get styles in firefox and ie